Lecture 15 - Innate Immunity and Viral Immune Evasion Flashcards
How long does it take for the innate immune system to be activated?
Fewer than 24 hours
Three types of PRRs
Toll-like receptors
Rig-like receptors
Nod-like receptors
Transcription factors activated by TLRs and RLRs
Interferon-releasing factor (IRFs)
AP-1
NFkB
Transcription factors activated when NLR detects PAMPs
AP-1
NFkB
NOD-like receptors that detect PAMPS
NOD1
NOD2
NLR that detects PAMPs, DAMPs, changes in cytosolic ionic milieu
NLRP3
What is activated when nod-like receptors detect DAMPS or cytoplasmic DNA?
Caspase-1 inflammasome
TLRs that detect viruses
TLR3
TLR7
TLR8
TLR9
Where are all TLRs that detect viruses located?
In the endosome
TLR3 ligand
dsRNA
TLR7 ligand
ssRNA
TLR8 ligand
ssRNA
TLR9 ligand
CpG unmethylated dinucleotides
How are TLRs activated?
Ligand induces dimerisation
TLR3 signal transduction pathway
1)
2)
3)
1) TRIF adaptor molecule
2) IRF3 and IRF7 lead to type 1 interferon release
3) NFkB leads to pro-inflammatory cytokine release
TLR8 signal transduction pathway
1) MyD88 adaptor molecule
2) NFkB leads to pro-inflammatory cytokine release
TLR7 and TLR9 signal transduction pathway
1) MyD88 adaptor molecule
2) IRF7 leads to type 1 interferon release
IRF3 effects 1) 2) 3) 4)
1) Acts as a transcription factor.
2) Stimulates type 1 interferon production.
3) Promotes ISG56 production.
4) Interacts with transcription factor eLF-3, leading to inhibition of protein synthesis.
Important rig-like receptors
RIG-I, MDA5
RIG-1, MDA5 location
Cytoplasmic
What is the adaptor molecule for RIG-1 and MDA5, and where is it located?
IPS-1, and located on the mitochondrial membrane (a MAV, mitochondrial-associated antiviral protein)
What does RIG-I recognise?
Short dsRNA.
5’ triphosphate on ssRNA. Cellular mRNA has a cap, so doesn’t have triphosphorylated 5’ end
MDA5 ligand
Long dsRNA
Examples of viruses detected by RIG-I
Influenza (orthomyxoviridae).
Vesicular stomatitis (rhabdoviridae)
Hep C, West Nile virus, dengue (flaviviridae)
Rotavirus (reoviridae)
Examples of viruses detected by MDA5
Encephalomyocarditis virus (EMCV) (picronaviridae)
Rotavirus (reoviridae)
West Nile virus, dengue (flaviviridae)
What are RIG-I and MDA5?
RNA helicases
What happens when RIG-I or MDA5 bind ligand?
Dimerisation, interaction with MAVS.
Interaction with MAVS facilitated via the CARD domain
What does activation of MAVS induce?
IRF pathway: Phosphorylation of IRF3 and IRF7 via stimulation of TRAF3 and TDK1.
NFkB pathway: MAVS interaction with TRAF6, RIP1 and FADD leads to NFkB activation.
Cytosolic PRRs that detect DNA 1) 2) 3) 4) 5)
1) TLR9
2) DAI
3) IFI16
4) AIM2
5) DHX9, DHX36
Example of a virus that releases DNA into cytoplasm
Poxviruses
Can viruses activated inflammasomes?
Yes.
Global actions of interferons
1) Brain - Induces IL-1, IL-6, TNFa, which lead to fever, fatigue, sleep
2) Induces CSFs, which lead to haematopoiesis, lymphocyte mobilisation
3) Liver - Induces IL-1, IL-6, TNFa, lead to acute phase protein production
Type 1 interferones
IFNa, IFNb
Type 1 interferon signalling cascade 1) 2) 3) 4)
1) IFNa/b binds to IFNa/b receptor on cel surface
2) Tyk2, Jak1 are phosphorylated
3) This leads to phosphorylation, dimerisation of STAT1, STAT2.
3) STAT1/2 dimer enters nucleus, joins with p48
4) STAT1/2/p48 trimer binds to ISRE (IFN stimulated response element)
Type 2 interferons
IFNg
Type 2 interferon signalling cascade 1) 2) 3) 4)
1) IFNg binds to IFNg receptor on cell surface
2) Jak1, jak2 are phosphorylated.
3) This leads to phosphorylation, dimerisation of STAT1.
4) STAT1 homodimer enters nucleus, binds to GAS (gamma activation site)
Are interferon stimulated response element genes produced at any time except during viral infection?
No
Examples of IFN-induced proteins 1) 2) 3) 4)
1) MxA
2) OAS
3) RNase L
4) Protein kinase R (PKR)
MxA protein 1) 2) 3) 4)
1) Found only in the cytoplasm
2) Recognises, sequesters viral nucleocapsid in bundles.
3) Used as a marker of infection (only present in virus-infected cells)
4) Very effective against influenza
OAS and RNase L role
Detects and cleaves viral RNA species
OAS and RNase L mode of action 1) 2) 3) 4) 5)
1) Viral RNA detected, ISRE stimulated, OAS transcribed
2) OAS monomer forms a tetramer.
3) OAS tetramer makes 2’-5’-adenylic acid (nucleotide).
4) 2’5’-adenylic acid induces RNase L monomers to dimerise, making RNase L active.
5) Active RNase L degrades viral RNA
Which nucleotide does OAS tetramer make?
2’-5’-adenylic acid
What induces dimerisation of RNase L monomers?
2’-5’-adenylic acid
Antiviral protein that shuts down host cell protein translation
Protein kinase R
Protein kinase R mode of action
1)
2)
3)
1) Inactive PKR has a dsRNA binding domain
2) If inactive PKR binds to dsRNA, PKR is phosphorylated, dimerises, is activated
3) Active PKR phosphorylates host transcription factor elF2a, shutting down host translation
How do poxviruses subvert host antiviral response?
Inactivate adaptor molecules of TLRs, so that they can’t initiate signal transduction
Viruses that alter TLR adaptor molecules to subvert host antiviral response
Poxviruses, hep C
Way that influenza can subvert host antiviral response
Makes ‘fake’ dsRNA that can bind to receptor (RLR, TLR3), bit can’t initiate downstream activity
Ways that viruses can subvert host antiviral response 1) 2) 3) 4) 5) 6)
1) Inactivate TLR adaptor molecules
2) Make ‘fake’ dsRNA that can bind to receptor, but can’t initiate downstream activity (influenza)
3) Interfere with helicase activation (poliovirus)
4) Interfere with MAVS signalling (Hep C)
5) Prevent activation of transcription factors (rabiesvirus)
6) Prevent MHCI presentation
Example of a virus that interferes with helicase activaiton
Poliovirus
Example of a virus that interferes with MAVS signalling
Hep C
Example of a virus that prevents activation of transcription factors
Rabiesvirus
Viruses that interfere with IFN signalling 1) 2) 3) 4)
1) West Nile virus
2) SARS coronavirus
3) Measles
4) Mumps
How does West Nile virus interfere with IFN signalling?
Prevents phosphorylation of Jak1 and Tyk2.
Redistributes cholesterol from the cell membrane, which disables IFN receptor
How does SARS CoV interfere with IFN signalling?
Prevents nuclear import of phosphorylated STAT1/STAT2 dimer
How do measles and mumps interfere with IFN signalling?
Prevent nuclear import of phosphorylated STAT1/STAT2 dimer
ADAR1 (p150)
1)
2)
3)
1) Detects dsRNA
2) Edits RNA by changing A to I
3) Results in destruction of RNA secondary structures, prevents RNA coding for anything
Antiviral protein that edits viral RNA
ADAR1 (p150)
Human immune effector proteins that detect dsRNA
1)
2)
3)
1) OAS/RNase L
2) PKR
3) ADAR1
Is it common for viruses to interfere with PKR function?
Yes. Almost every virus has a strategy for PKR interference
Examples of viruses that interfere with PKR by producing fake RNAs
HIV, adenoviruses, EBV
Example of a virus that interferes with PKR by degrading it
Poliovirus
Examples of viruses that interfere with PKR by preventing phosphorylation or dimerisation
Poxviruses
Reoviruses
Examples of viruses that itnerfere with PKR function by preventing phosphorylation of elF2a
Poxviruses
Hep C
Herpes simplex
